We have studied the electron transmission of an electrically conductive adhesive (ECA) composed of micro-size and nano-size metal fillers. By quantum mechanics, it has been shown that due to the quantization of electron states in the nano particle, energy barriers are formed at interconnects between the microfillers and nano particles. The energy barriers effectively reduce the electron transmission through the nano particles. For an ECA consisting of 50-nm-diameter nano particles, the electron conduction is reduced by a factor of 2, and the time for the electron to transmit through the nano particle is about 100 ps.